Nuclear fuel assemblies for light water nuclear reactors, such as, for example, pressurized water reactors (PWRs) and boiling water reactors (BWRs), generally include a plurality of fuel rods with circular cross-sections that are arranged parallel to one another at regularly or irregularly spaced intervals. Each fuel rod comprises a stack of fuel pellets (e.g., uranium oxide pellets) surrounded with cladding that is made from zirconium alloy or other suitable material. The fuel rods are held at the spaced intervals with respect to one another by one or more spacer grids. Each spacer grid includes a plurality of interlocking grid straps that are welded together to form an array of four-walled cells in an “egg-crate” configuration. A fuel rod may be contained within each of the four-walled cells. The entire fuel assembly typically has a square cross-section with a 14×14, 15×15, 16×16, or 17×17 array of fuel rods. One or more outer straps may encircle the periphery of each spacer grid.
FIG. 1 presents a partial perspective view of a conventional spacer grid 100 for a fuel assembly 102. The spacer grid 100 includes two sets of perpendicularly placed grid straps 112, 114. Each grid strap 112, 114 includes a series of regularly spaced notches (not shown) that allow for the assembly and interlocking of the grid straps 112, 114 to form an array of four-walled cells 116. Each four-walled cell 116 contains four intersections 118. The grid straps 112, 114 may be welded together at these intersections 118. The purpose of the four-walled cell 116 is to support a single fuel rod 130 (FIG. 2) in the square array of the fuel assembly 102. The periphery of the grid straps 112, 114 may be encircled with one or more outer straps 120. A plurality of springs 122 and a plurality of dimples 124 are integrally formed on, or attached to, the grid straps 112, 114 and extend inwardly within each four-walled cell 116. The springs 122 and dimples 124 provide support structures for contacting the fuel rod cladding and holding it within the four-walled cell 116.
FIG. 2 presents a cross-sectional top plan view of one of the aforementioned four-walled cells 116, with a fuel rod 130 contained therein. As shown, the springs 122 and dimples 124 extend inwardly within the four-walled cell 116 to engage and provide support for the fuel rod 130, as previously discussed.
When the PWR or BWR is in use, a coolant, such as for example, water, flows from the bottom of the fuel assembly upwards through the spaces between the fuel rods. The temperature of the coolant varies as it travels upwards, absorbing thermal energy from the fuel rods. At locations adjacent to the fuel rods, the coolant may be partially overheated, which can adversely affect the thermal performance of the fuel assembly and reduce the output power of the fuel rods. One way of alleviating these partially overheated regions is to design the spacer grids to more effectively deflect and mix the coolant as it flows upwards through the fuel assembly, thereby promoting a more uniform distribution of coolant temperature. Such a design can be accomplished by attaching “mixing vanes” to the top, downstream portion of the grid straps that comprise the spacer grid, as shown. The mixing vanes are intended to promote the flow of coolant in a lateral direction as well as a longitudinal direction along the fuel rod axes. This flow pattern allows the coolant to more effectively move between the fuel rods, and between the lower temperature regions and the partially overheated regions of the fuel assembly.
FIG. 3 presents a partial perspective view of a grid strap 140 and conventional PWR mixing vane 142, wherein the mixing vane 142 is disposed at the top, downstream portion of a cell wall 144. It should be noted that although they are generally similar in configuration to the PWR vane shown, BWR vanes are typically much smaller in size. Other conventional mixing vane designs are disclosed in U.S. Pat. No. 5,440,599 to Rodack et al., U.S. Pat. No. 6,807,246 to Kim et al., U.S. Pat. No. 3,862,000 to Pugh et al., U.S. Pat. No. 4,758,403 to Noailly, U.S. Pat. No. 5,299,245 to Aldrich, U.S. Pat. No. 5,283,821 to Karoutas, U.S. Pat. No. 6,606,369 to Smith III et al., U.S. Pat. No. 6,278,759 to Yoon et al., U.S. Pat. No. 4,692,302 to DeMario et al., U.S. Pat. No. 5,265,140 to Perrotti, U.S. Pat. No. 6,236,702 to Chun et al., and U.S. Pat. No. 5,339,341 to King et al. Nozzle-type mixing vanes are disclosed in U.S. Pat. No. 4,726,926 to Patterson et al. and U.S. Pat. No. 6,130,927 to Kang et al.
Conventional mixing vanes, however, tend to be restricted in the ways in which they deflect the flow of coolant as it moves upwards through the fuel assembly. They do not provide a robust means for adjusting or tuning the vanes in order to optimize the flow pattern that is formed. As a result, conventional mixing vanes cannot effectively achieve the most desirable type of coolant flow—even and sustained mixing—for the particular application at hand. Thus, there exists a need for a new type of mixing vane that guides the coolant in a desired flow pattern to more effectively mix the coolant as it moves upward through the fuel assembly.